To better understand the normal regulation of IgA expression and its dysfunction in disease, two related issues will be examined. The first concerns when during normal ontogeny do B cells expressing IgA arise, and whether they are competent to differentiate into IgA secreting plasma cells, or are they functionally immature. The ontogeny of IgA bearing B cells, an area where conflicting data abound, will be examined using three approaches: i) quantitative immunofluorescence analysis using monoclonal antibodies specific for human heavy and light chain isotypes, ii) biochemical analysis of surface immunoglobulins and iii) in situ hybridization to enumerate cells containing alpha mRNA transcripts. Once the time of appearance of IgA B cells has been unequivocably established, a variety of polyclonal B cell stimulators will be used to compare the differentiation requirements of these early appearing B cells with those from adults. In addition, Epstein-Barr virus will be used to transform IgA B cells from neonatal and adult blood and colostrum. The ability of IgA B cells from the different sources to be transformed, and the IgA subclass distribution in the transformants will be determined. Parallel analysis of patients with IgA deficiency may reveal informative differences from normal individuals of comparable age. These in vitro findings will be compared with normal in vivo events by enumeration and subclass distribution of circulating IgA plasma cells as a function of age. The second issue concerns how the differentiation on IgA B cells is regulated, specifically what is the role of cells bearing receptors for the Fc portion of IgA (Fc alpha R). Quantitative assays for the identification of cells bearing Fc alpha R will be developed. These assays will be used in conjunction with monoclonal antibodies specific for the various hematopoietic lineages to evaluate the cell type and tissue distribution of Fc alpha R-bearing cells in normal individuals and patients. Cell lines expressing Fc alpha R will be identified, and the inducibility of Fc alpha R on normal and transformed cells will be determined. Both normal and transformed cells will be used as a source of Fc alpha R for biochemical characterization, as a source of immunogen for the production of monoclonal antibodies to the Fc alpha receptor(s), and to determine if the same Fc alpha R is expressed on different cell types. These antibodies will simplify screening for Fc alpha R bearing cells, and more importantly, should be useful in determining the normal function of these cells, and how this receptor function may be disturbed in diseases involving IgA antibodies or the lack of them.